Modern cameras feature a built in light meter. The meter measures the intensity of the light that is being reflected by objects in the vista.
Modern cameras feature built-in chip logic to interpret the light meter data. Algorithms (math formulas) then send signals that control functions such as shutter speed, aperture setting and ISO (sensitivity to light). All this action is happening as you compose and focus. Thus chip logic is continuously changing these setting based on conditions.
Because even subtle camera movements, zoom changes, lighting changes, or subject movement will cause data changes – sometimes this results in camera setting swings that might render the setting substandard.
As your skills improve you will recognized that perhaps you might get better results if you override the continually changing data. Perhaps it will be best if you set the exposure by metering a different segment of the vista and then use this data to set the camera.
To accomplish, we swing the camera, as we compose, select that area to meter and then partially depress the shutter release. This action locks the meter reading averting updates. Such technique is an acquired skill that refines our abilities to take great pictures.